Extraordinary footage of a rarely seen giant deep sea fish has been captured by scientists.

Using a remotely operated vehicle, they caught a rare glimpse of the huge oarfish, perhaps the first sighting of the fish in its natural setting.

The oarfish, which can reach 17m long, has previously only been seen on a few occasions dying at the sea surface, or dead washed ashore.

The scientists also filmed for the first time the behaviour of a manefish.

Mark Benfield from Louisiana State University, Baton Rouge, US was undertaking a survey as part of the Serpent project, a collaboration between marine scientists and energy companies such as BP, Shell, Chevron and Petrobras working in the Gulf of Mexico.

Using remotely operated vehicles (ROV) provided by the oil and gas companies, the scientists are able to explore the deep like never before.

During one of these surveys, the scientists glimpsed a giant oarfish.

Sea serpent

Oarfish (Regalecus glesne) are one of the world’s longest fish reaching 17m.

Their strange appearance may have provided the basis for the sea serpent myths told by early ocean travellers.

Not only are they elongated, they also have a prominent dorsal fin which gives it an unusual “serpent” appearance.

Recalling the event Professor Benfield explained how at first, they thought the fish was simply a drilling pipe called a riser being lowered into the water.

“We saw this bright vertical shiny thing, I said ‘are they lowering more riser?’ as it looked like they were lowering a huge pipe.”

“We zoomed in a little bit and we said ‘that’s not a riser that’s a fish!'”

“As we approached it retreated downwards swimming tail first in a vertical orientation as the ROV followed,” Professor Benfield explained.

The team followed the fish for about five minutes before breaking off contact to resume their surveys.

“What was interesting about the fish was its swimming behaviour,” said Professor Benfield.

“It moved by undulating its dorsal fin in waves that propelled it backwards at quite a good speed.”

Early estimates measure the fish at between 5m and 10m in length.

Filmed alive

Professor Benfield said this may be the first time the oarfish has been filmed alive swimming in the so-called mesopelagic layer of the ocean.

Usually, they are seen dying at the sea surface or washed up dead.

The fish may have been caught on camera at a depth of 765m at another Serpent survey site, off western Africa in 2007, but a positive identification has not yet been made from that video.

On this occasion the fish was observed underneath Thunderhorse in the Gulf of Mexico, one of the largest semi-submersible oil rigs in the world.

The Serpent project run by the National Oceanography Centre, Southampton (NOCS) is a unique collaborative project between scientists and industry.

Oil and gas companies allow scientists access to their deep sea technologies and infrastructure in a bid to aid their research.

“(It) provides a wonderful opportunity to learn more about life in the depths of the Gulf of Mexico. That we found an oarfish while doing so was a fantastic bonus,” said Professor Benfield.

The oarfish was not the only new discovery the team has made.

On another ROV survey in the Gulf of Mexico, the team came across the deep sea manefish (Paracaristius sp.).

They report this sighting in the journal Copeia.

This observation enabled the scientists to get a rare insight into the behaviour of the fish in its natural environment.

That gave them a further understanding of how it lives and what it looks like.

“When you see manefish collected in trawls they are incredibly beat up, they don’t look like much of a fish at all,” said Professor Benfield.

“So to actually see this fish in its natural habitat with its fins beautifully splayed out almost as a parachute, we get an idea that it’s a really good swimmer.”

Manefish are thought to steal food from or feed on a jellyfish-like animal called a siphonophore.

This may explain why the fish needs to swim so accurately.

“This fish has very precise control over its orientation and position, so if you are in very close proximity to an animal with tentacles that could potentially damage you, you want to able to really precisely control your locomotion.”

Professor Benfield is excited by the potential for further discoveries and revelations from the deep that the Serpent project may bring.

“It’s all very exciting, my vision for the Gulf Serpent Project is to establish a Gulf-wide deep sea biological observation system, with hundreds of ROV-equipped ships and rigs in the deep Gulf.”

“(We can) get a good idea of what species are present, where they are present, and what are they doing.”

This claim of 17 m is really higly dubious. Even the often claimed maximum length of 11 m seems quite obsure, and I have never seen any photos of an oarfish which came only close to this. It seems this is just another among the many big-fish-stories which became popular.

I checked against several sources, including an old book I own (it was published by the NHM Vienna) which only deals with oarfish, including relations to sea serpents. It was stated in the book, that the largest actually known specimens were around 6 m in length (including a preserved specimen in the museum I have seen myself some years ago), and that there was a story about an 8 m long oarfish, but there seems to be no hard facts for this case. The largest preserved oarfish is “only” 7.3 m in length, and it is really huge compared to most of the known specimens, and it is really hard to imagine that this species can grow much larger. Even the often cited 11 m are quite dubious, not to speak about everything above. I found also that there are several wrong informations about the actual size of several specimens. There was for example a really wonderful and very large specimen found at 2007. There are many photos of it, including one which shows many men posing with it. The length was stated to be 9 m. I checked the photo and compared the size of the largest man on the photo with the actual length of the fish, and it turned out that this oarfish was in fact only around 6 m.